It is not, perhaps, possible to prevent inundations of
this kind altogether, but it is the duty of good princes to afford all possible assistance. The assistance required is to
prevent excessive overflow by the construction of dams, and
to obviate the filling of rivers, produced by the accumulation
of mud, by cleansing the canals, and removing stoppages at
their mouths. The cleansing of the canals is easily performed,
but the construction of dams requires the labour of numerous
workmen. For the earth being soft and yielding, does not
support the superincumbent mass, which sinks, and is itself
carried away, and thus a difficulty arises in making dams at
the mouth. Expedition is necessary in closing the canals to
prevent all the water flowing out. When the canals dry up
in the summer time, they cause the river to dry up also;
and if the river is low (before the canals are closed), it cannot supply the canals in time with water, of which the country, burnt up and scorched, requires a very large quan-
tity; for there is no difference, whether the crops are flooded
by an excess or perish by drought and a failure of water.
The navigation up the rivers (a source of many advantages)
is continually obstructed by both the above-mentioned causes,
and it is not possible to remedy this unless the mouths of the
canals were quickly opened and quickly closed, and the canals
were made to contain and preserve a mean between excess
and deficiency of water.